Do not cover the actor with so much dirt that the weight of it
restricts his movement.
I have to echo this. I was buried for a movie. I had a box over my head (also covered with dirt), with a vacuum hose for an air tube. The dirt was damp and I could hardly move. No one could hear me call for help, because the dirt is almost soundproof.
Rik's advice is good, but pay careful attention.
Another way to do it is put your actor in the hole and cover it with a sheet of wood. BE SURE TO USE 2" by 4" (on their side, not flat) or 4" by 4" CROSS SUPPORT BEAMS across the hole and lay the sheet board on top of them. Otherwise, the weight of the dirt can collapse the sheet of wood. Have a hole (big enough to crawl out of) on one side of the board. Your actor should use the side of the hole and not put his weight on the wood, when he comes out.
Lay a sheet of plastic over the board and hole, and cover it all with thick dirt. You can use a walkie talkie or cell phone to relay "Action" to the actor under the ground. He can cut or rip the plastic (which should sag a bit into the hole from the weight of dirt). The dirt falls in (like into the space of the casket) and the zombie pushes out.
If you put a fog machine in with your actor, he can also pump a small amount of fog in the hole, just prior to coming out.
I buried my wife using the above method and it worked great.